Microscopic Approaches to Physisorption: Theoretical and Experimental Aspects

1 Introduction

The writing of scientific reviews can be regarded as a service to the scientific community as well as a beneficial, though time-consuming, undertaking for the authors. There can be several purposes of such a review. First, it can be viewed as a collective reference source to aid workers in the field being reviewed and in related subjects in obtaining information about current developments. Secondly, it can be used as a means of forming a coherent unified approach to a certain set of problems, thus bringing together various aspects of the subject under study. Thirdly, it can be constructed such as to introduce the field to workers in other areas and hence serve as a cross-fertilization agent. We have here attempted the impossible, namely, to achieve all three purposes.

The study of adsorption phenomena is one of the oldest branches of physical chemistry (for a comprehensive review of early literature see ref. 1), starting with the observations by Scheele (1773) and Fontana (1777) of the uptake of gases by charcoal, and the discovery by Lowitz (1785) of the discoloration of solutions by charcoal. The distinction between regimes of adsorption phenomena, namely physical and chemical adsorption, was recognized early on and was often associated with the conditions leading to one adsorption class or the other. Among the names used to describe physical adsorption are van der Waals adsorption, low-temperature adsorption (versus high-temperature adsorption), secondary adsorption (versus primary adsorption), and capillary condensation, implying that capillary forces are responsible for physical adsorption. More modern definitions are based on the theory of chemical binding, identifying physisorption as the state of interaction between a n atom or molecule and a sur face, where no chemical bonds arc formed by charge rearrangement or sharing of electrons. Related to the above is the distinction of classes of adsorption according to the magnitude of the interaction energy (electron volts for chemisorption versus fractions of electron volts for physisorption). Modern spectroscopic techniques, especially ultraviolet photoemission spectroscopy (UPS), provide new diagnostic methods for distinguishing between the above adsorption regimes, as discussed in Section 3 (p. 34).

The traditional, and until quite recently the only, methods used for the study of adsorption phenomena were thermodynamical in nature. In this respect, theoretical efforts towards microscopic models of the interaction preceded the development and application of microscopic experimental tools. The main microscopic theoretical approaches are reviewed in Section 2. These studies are complemented by statistical mechanical treatments of the interaction of gases with surfaces, which have been reviewed extensively recently. Section 3 covers microscopic experimental approaches for the study of physisorption. In order to satisfy our 'third review criterion', the discussion of the results obtained by each of the experimental techniques is preceded by a brief exposition of the physical principles underlying the methods and the conventions and terminology employed. We have attempted throughout to review the methodology and results of recent studies and to indicate the link between the data and microscopic theoretical models.

Section 2 contains three main parts in which we discuss the general theory of van der Waals forces and physical adsorption (p. 5), semi-empirical calculations (p. 19), and the underlying principles and results of microscopic theories of physisorption (p. 21). In the last we have concentrated mainly on a discussion of our own theoretical studies.

Section 3 is divided into four main parts. In the first (p. 33) a classification of the methods is followed by a discussion of the use of u.v. photocmission in establishing a spectroscopic criterion for the definition of a physisorption system. Experimental methods for the study of atomic arrangement (LEED, neutron scattering, molecular beam scattering) are discussed and the results of recent studies demonstrated in the second part (p. 39). In addition the use of the above techniques for investigations of dynamical properties of physisorbed atoms (p. 61) and the atom-surface interaction potential (p. 77) are discussed. In the third subsection (p. 81) experimental techniques for the study of electronic structure are discussed (UPS, XPS, and field emission techniques). The review of the results is accompanied by theoretical arguments. Finally, we discuss the application of electron spectroscopy methods (LEED and Auger) for the measurement of adsorption isotherms (p. 97).

We conclude the review, in Section 4, with a brief summary and prognosis.

2 Theoretical Approaches for the Study of Physisorption

Introduction. — The basic theoretical problem in adsorption studies is the calculation of the wavefunctions of molecules which are interacting with a solid surface as well as with each other. The procedure which offers the greatest opportunity for insight into the physics of the process is that of first calculating the potential energy of a single molecule interacting with the solid surface and then determining its quantum and statistical mechanical properties in this potential field. The larger problem of interaction among adsorbed molecules is much more complicated and much less progress has been made. In general, it is obvious that the potential energy of interaction is fundamental.

We are concerned with the calculation of the potential energy [??] of a neutral molecule at position r which is interacting with a solid surface through forces associated with physical adsorption. The definition of these forces as distinct from those involved in chemical adsorption (which are caused by chemical interactions) is vague. It is known, however, that a neutral molecule far from a solid surface interacts through attractive van der Waals, or dispersion, forces. Dispersion forces may be described as physical in that they involve no chemical interactions such as charge transfer or rearrangement. From a...